1. Field of the Invention
The present invention relates to a process and to a system for manufacturing an encapsulated semiconductor device; in particular, the ensuing treatment will make reference, without this implying any loss of generality, to the production by molding of a power package for a semiconductor device, of the full insulated type.
2. Discussion of the Related Art
Power semiconductor devices, for example power MOSFETs, are known that comprise a plastic package designed to encapsulate a die of semiconductor material integrating a corresponding integrated circuit, wherein the plastic package is commonly obtained by molding.
For example, FIG. 1a and 1b show a semiconductor device 1 (in particular a power device) encapsulated in a package 2, made of plastic material, for example epoxy resin, of the type known as JEDEC TO-220. The semiconductor device 1 comprises a die 3 of semiconductor material and a leadframe 4 set at least partially within the package 2 and designed to support the die 3 within the same package 2, and to provide the electrical connection towards the outside of the integrated circuit within the die 3. The leadframe 4 comprises: a metal plate (known in general as “die pad”) 5, set entirely within the package 2 and having a top surface 5a, to which the die 3 is coupled (for example, via interposition of adhesive material); and a plurality of leads 6, for example three, which come out of the package 2. In a way not illustrated, the die pad 5 is made of a single piece with one of the leads 6 (in particular, with the lead set in a central position), consequently constituting an electrode of the semiconductor device 1, and the die 3 is connected electrically to the remaining leads 6 by means of bond wires, which extend from a respective contact pad, carried by a top surface of the die 3 not in contact with the die pad 5, and a respective lead 6. The package 2 moreover has a through hole 7 at an end portion thereof (opposite to the one from which the leads 6 come out), for coupling, for example, by means of a screw or rivet, of the semiconductor device 1 to a heat sink (not illustrated). In this regard, the die pad 5 transfers the heat generated in use by the circuit integrated in the die 3 towards the aforesaid heat sink.
Given the need to ensure a good transfer of heat towards the heat sink, and (at least in the case of insulated packages) to insulate the die pad 5 electrically from the outside of the package 2, during manufacture of the semiconductor device 1, and in particular of molding of the package 2, a controlled thickness of the encapsulating material of the package underneath the die pad 5 needs to be guaranteed (in particular of the material in contact with a bottom surface 5b of the die pad 5, opposite to the top surface 5a to which the die 3 is coupled). The thermal and electrical performance of the resulting semiconductor device 1 can vary even considerably according to the aforesaid thickness, which is markedly dependent on the technique for manufacturing the package 2, and in particular on the correct positioning of the leadframe 4 during the molding step. An incorrect alignment of the leadframe 4 with respect to the mold used for the formation of the package 2 can cause a degradation of the thermal performance (in terms of thermal resistance Rth) if thickness of the encapsulating material is greater than an upper specification limit (USL), or the exposure of the bottom surface 5b of the die pad 5 if thickness of the encapsulating material is considerably lower than a lower specification limit (LSL). If the aforesaid thickness is smaller than the lower specification limit, an inadequate flow of encapsulating material during molding may also occur, causing the creation of voids at the backside of the package 2.
In a known manner, according to the molding technique and the resulting structure of the package, power packages for semiconductor devices are divided into “full molded” and “full insulated.” In both cases, the die 3 is entirely coated with the encapsulating material, but in full molded packages areas of exposed metal may remain (for example, portions of the die pad 5 may be accessible from the outside of the package 2), whereas in full insulated packages the total absence of exposed metal needs to be guaranteed. It is evident that, especially in the case of full insulated packages, the presence of an excessively thin layer of encapsulating material on the backside of the device can irreparably jeopardize its performance.
It is consequently necessary to arrange the leadframe 4 and keep it in a proper and pre-set position within a corresponding mold during the step of molding of the package 2, in particular during injection of the encapsulating material and its subsequent hardening (polymerization). In the past, a wide range of molding processes have been proposed, designed to address this need.
For example, one of the proposed techniques envisages the use of fixed pins (so-called “fixed-pin” technique), fixedly coupled to the mold, and such as to come to abut on opposed portions of the top and bottom surfaces of the die pad 5 within the molding cavity, thus keeping the die pad in a desired position upon closing of the mold. Once the molding step is terminated, as illustrated in FIG. 2, the package 2 has, however, voids 10, which leave the die pad 5 exposed, in positions corresponding to the ones occupied by the fixed pins during molding. This technique can consequently be used for the production of full molded packages, but not for the production of full insulated packages.
In the case where the production of a full insulated package is required, the process described previously can be completed with a final off-line step (i.e., one distinct from and subsequent to the molding step) of filling of the voids 10 left by the fixed pins, with an epoxy compound (commonly known as “potting”), which is subsequently cured. FIG. 3 shows the resulting semiconductor device 1, in which reference 11 designates the filling portions that totally close the voids 10. The resulting process suffers, however, from a series of drawbacks, amongst which: the longer duration of the manufacturing method; the need for additional equipment and the associated additional costs; and the possibility of occurrence of reliability problems due to the fact that the epoxy compound is a material “external” to the encapsulating material that forms the body of the package.
To overcome these drawbacks, an alternative molding method has been proposed, which envisages the use of retractable ejector pins. In detail, in an initial step, FIG. 4a, the leadframe 4 is inserted within a molding cavity 12 of a mold 13. An input channel (or gate) 14 is in fluid communication with the molding cavity 12 and enables the introduction of encapsulating material. The leadframe 4 is kept in a desired position by means of the use of retractable pins 15, which are brought into contact with the die pad 5 (as highlighted by the arrows in FIG. 4a) so as to abut on a respective top surface 5a or bottom surface 5b thereof. For this purpose, guides 16 are provided in the mold 13, and the retractable pins 15 can slide within the guides 16 by the action of suitable actuators (not illustrated). Subsequently (FIG. 4b), encapsulating material 17 is injected within the mold 13; the retractable pins 15 keep the die pad 5 in the proper position so as to guarantee the thickness required by specifications of the encapsulating material 17 on the backside of the package 2. Next (FIG. 4c), once the molding cavity 12 is entirely filled with the encapsulating material, but before it polymerizes, the retractable pins 15 are retracted and moved away from the die pad 5 (causing them to slide again in the guides 16, as indicated by the arrows in FIG. 4c). The pressure of injection of the encapsulating material 17 is then increased so that it comes to occupy the empty spaces left by the retractable pins 15, and subsequently the required compactness of the package is achieved by hardening.
This method enables formation of a full insulated package with a good accuracy of the thickness of the encapsulating material 17 on the back of the leadframe 4. However, it has the problem of rapid wear of the retractable pins 15 and of the corresponding guide within the molding cavity 12, due to the abrasive characteristics of the encapsulating material 17 used for the manufacturing of the package 2 (in general, epoxy resin containing an inorganic part, known as filler, with abrasive properties). This aspect has a negative impact both on the costs of production and on the quality of the devices thus produced.